The present invention relates to a method and apparatus for making small spherical particles. More particularly, the present invention relates to an inexpensive means to produce small solid spherical particles such as can be used in a magnetic particle display panel disclosed in my copending patent application Ser. No. 566,455 filed Apr. 9, 1975 now abandoned.
The magnetic particles required for use in the magnetic display panel disclosed in my above mentioned copending patent application must be substantially spherical in shape and of uniform size. A large number of these particles would be required in the display panel to provide desired resolution. It is therefore desirable to produce spherical particles of substantially uniform diameter in great quantities and at low cost. The magnetic particles required for the magnetic particle display panel are typically less than 200 microns in diameter.
Most of the techniques proposed in the past for generating solid particles are limited by the control which can be exercised over the physical parameters of the particles generated, especially when the parameter in question is particle uniformity. One system proposed in the past is a liquid droplet generator which makes use of the so called Rayleigh mechanism. With this mechanism, droplets are generated by the electromechanical excitation of a liquid jet issuing from an orifice, resulting in the break up of the jet into drops. The drops are formed in a single stream and are easily controlled, equally spaced, and exhibit a remarkable degree of uniformity in size. The Rayleigh mechanism has been used for generation of solid spheres of approximately 200 microns diameter by breaking up a jet of material in its molten state and solidifying it by cooling after the particles are formed. This method, however, is limited by difficulties in producing droplets of highly viscous fluids and by the collection of impurities at the capillary openings when very small capillaries are used. Furthermore, this method does not produce suitable particles when liquids containing a high concentration of solid particles which have a tendency to settle out are used. It therefore becomes rather difficult to produce solid spheres of small size with ease.
Also in the past there have been methods proposed for forming filamentary materials from a molten material. A stream of the molten material is directed onto an outer circumferential boundary of a rotating member having grooves or indentations formed in the outer circumferential boundary. The rotating member serves as a chill block to remove the heat from the molten material thereby solidifying the material and forming it into solidified filaments. The movement of the chill block is disposed to effect the ejection of the solidified molten stream from the grooves or indentations in the form of filaments by centrifugal force. However, it should be recognized that these methods do not lend themselves to making small spherical particles.
Spherical particles of polyethylene with carbon-black filler (10% by weight) have been made by emulsifying ultrasonically polyethylene and carbon-black in a boiling solution of water, glycerol, and sodium lauryl sulfate. Then quenching the emulsion by pouring quickly into water and stirring rapidly. The particles ranged over a wide distribution of sizes. Furthermore, this method would probably not work well when a high concentration of filler material is needed, or when the filler material separates easily from the binder.
In view of the foregoing, it should now be understood that it would be desirable to provide an improved method and apparatus for producing small spherical particles that would solve the above and other problems.
Accordingly, one of the objects of the present invention is to provide an apparatus for making small solid particles of uniform size and/or of uniform density.
Another object of the invention is to provide an improved apparatus that employs a heated disc to make small solid spherical particles and eliminates the requirement of capillary tubes which are subject to clogging.
Yet another object of the present invention is to provide a method that employs relatively inexpensive and easily operable equipment to make small solid particles.